Modeling the effects of external nutrient reductions on algal blooms in hyper-eutrophic Lake Taihu, China

Harmful algal blooms in Lake Taihu, China, are getting plagued due to excess nutrients loads. Despite several recent strict nutrient management strategies, especially to reduce the external nutrient loads, Lake Taihu still receives large amounts of nutrients from its tributaries. As a result, frequency and intensity of algal blooms have not abated as expected. In this study, a three-dimensional eutrophication model was used to identify the limiting factors for algal growth and the effect of external nutrient loads reduction on water quality. The modeling results showed that the limiting factor for the dominant algae (cyanobacteria) depended on seasonality and location. Water temperature affected algal growth in winter, while light was important for deeper water columns in all seasons. Phosphorus (P) and Nitrogen (N) impacted algal growth from spring to autumn. Results showed that Chlorophyll a (Chl-a) concentrations only decreased about 38%, 26% and 24% in Meiliang Bay, Zhushan Bay and Northwest Zone, respectively when implementing high nutrient reduction scenario (external N and P inputs reduced up to 80% and 50%). External nutrient reduction for both N and P is a prudent and likely most practical solution for reducing bloom potentials and overall trophic state of this currently hypereutrophic system. However, achieving observable algal control is not likely in the short term and other ecological restoration tools will be required to battle with the legacy of several decades of nutrient over-enrichment.